Jordan was inspired to specialize in ichthyology by the Swiss-born American naturalist Louis Agassiz. From the 1870s, Jordan made extensive field trips in the United States and its coastal areas, and his efforts led to the naming of a total of 1,085 genera and more than 2,500 species of fishes, together with broader classifications of them. With B.W. Evermann he wrote the authoritative “The Fishes of North and Middle America,” and his Manual of the Vertebrates of the Northern United States went through 13 editions between its publication in 1876 and 1929. Jordan devoted his later career mainly to the cause of international peace, acting as chief director of the World Peace Foundation.